Confirm the coordinate frame for Fermi RA/dec

[ayshih]

In sunpy.instr.fermi, both get_detector_sun_angles_for_time() and get_detector_sun_angles_for_date() call sunpy.sun.sun functions to get the Sun's right ascension and declination. They use the functions for the apparent RA/dec referenced to the equinox of date, but I suspect that Fermi reports the spacecraft RA/dec (and hence the detectors' RA/dec) as astrometric RA/dec referenced to the J2000.0 epoch. (With sunpy/sunpy#3137, we can now provide precisely that RA/dec.) The coordinate frame used by Fermi needs to be confirmed, and the RA/dec calls changed if necessary.

Ping @aringlis

[aringlis]

@ayshih I'll look into this.

[aringlis]

@ayshih, the headers for the Fermi spacecraft pointing files include the following entry:

EQUINOX = 2000. / equinox for ra and dec

So it looks like you're correct that the functions in sunpy.instr.fermi need to be changed.

[ayshih]

Great! You also need to confirm whether the RA/dec are astrometric or apparent. That is, whether spacecraft motion is taken out (astrometric) or not (apparent).

[aringlis]

That I can't figure out from the information in the pointing files. I'll send a query to the Fermi helpdesk, that's probably the easiest solution.